Just write a subclass to the class on which you call the ->prepare method. In the subclass, add the prefix to the string, and re-dispatch to superclass method with $self->SUPER::prepare(@arguments)

Then in the new_dbh sub you can rebless the created statement handle with bless.

This should work, but it's an ugly hack, and should only temporarily used for debugging.

Update: here's a code example (untested, but should work roughly like this):

#!/usr/bin/perl
use strict;
use warnings;
{
    package My::DBI::st;
    our @ISA = qw(DBI::st);

    sub prepare {
        my ($self, $str, @args) = @_;
        $self->SUPER::prepare("/* userid $ENV{USER} */ $str", @args);
    }
}

sub new_dbh {
    ...
    my $dbh = DBI->connect(...);
    return bless $dbh, 'My::DBI::st';
}
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